1. Field of the Invention
The present invention relates to an electrical leak detecting apparatus for detecting electrical leak of a power supply device.
2. Description of the Prior Art
One example of conventional electrical leak detecting apparatuses is shown in FIG. 20. In this prior art electrical leak detecting apparatus, voltage division resistance elements R1 and R2 each having a high resistance value are connected to each other in series between output terminals of a DC power supply E acting as a mechanical power source of an electric vehicle and a detection resistance element R3 is connected between a junction of the voltage division resistance elements R1 and R2 and ground (vehicle body) such that electrical leak is detected by fetching, as a detection voltage, a voltage drop occurring between opposite ends of the detection resistance element R3 as disclosed in, for example, Japanese Patent No. 3307173 (2002).
The prior art electrical leak detecting apparatus is operated as follows. Since the DC power supply E used as the mechanical power source of the electric vehicle outputs a quite high voltage of about 200 to 300 V, the DC power supply E is electrically isolated from the vehicle body, i.e., is in a floating state such that a man does not receive an electric shock even if the man comes into contact with the vehicle body. However, in case dielectric breakdown happens between a high voltage system including the DC power supply E and ground, the man may receive an electric shock through establishment of a current path upon contact of the man with the vehicle body or the like. On the other hand, since the high voltage system is isolated from ground, electric current does not flow unless the man comes into contact with the high voltage system even if the dielectric breakdown happens, so that it is impossible to detect electrical leak. Thus, the prior art electrical leak detecting apparatus is adapted to detect electrical leak prior to contact of the man with the high voltage system.
FIG. 21 shows a state of the above prior art electrical leak detecting apparatus, in which dielectric breakdown happens between a negative polarity of the high voltage system and ground and a man is held in contact with the high voltage system. In FIG. 21, a resistance r represents a resistance at a location of the dielectric breakdown between the high voltage system and ground, i.e., a dielectric breakdown resistance and a resistance R represents a resistance of a human body. It is supposed here that the DC power supply E has an output voltage V and the voltage division resistance elements R1 and R2, the detection resistance element R3, the dielectric breakdown resistance r and the human body resistance R have resistance values R1, R2, R3, r and R, respectively. If the resistance values R1 and R2 of the voltage division resistance elements R1 and R2 are far larger than the resistance value r of the dielectric breakdown resistance r, a leakage current (ground fault current) I flowing through the human body resistance R is expressed by the following equation (1).I=V/(r+R)  (1)
Meanwhile, the human body resistance R may vary according to environments such as humidity. In case the human body resistance R is set to zero, the leakage current I reaches a maximum.
On the other hand, if a detection voltage V1 across opposite ends of the detection resistance element R3 at the time the man is held out of contact with the high voltage system, namely, the resistance value R of the human body resistance R is infinite is obtained on the supposition that the resistance values R1 and R2 of the voltage division resistance elements R1 and R2 are larger than the resistance value R3 of the detection resistance element R3, a leakage current i flowing through the voltage division resistance element R1, the detection resistance element R3 and the dielectric breakdown resistance r via ground is expressed by the following equation (2) and the detection voltage V1 across the opposite ends of the detection resistance element R3 is given by the following equation (3).i=V/(R1+R3+r)  (2)V1=V×R3/(R1+R3+r)  (3)
Since the detection voltage V1 corresponding to the leakage current I is obtained by substituting the equation (1) for the equation (3), electrical leak can be detected from this detection voltage V1.
In the above prior art electrical leak detecting apparatus, since presence or absence of occurrence of electrical leak can be detected but a location of occurrence of electrical leak cannot be detected, it is difficult to promptly take a proper countermeasure against electrical leak.
Furthermore, in the above prior art electrical leak detecting apparatus, in case dielectric breakdown happens in a transformer for insulating an input side and an output side of a DC/DC conversion circuit of a power supply device (not shown), other appliances connected to the DC power supply E may be damaged upon application of a high voltage of a secondary winding of the transformer to a primary winding of the transformer.